The present invention relates to a method for charging a nonaqueous secondary battery such as a lithium ion secondary battery or the like.
Generally, when a nonaqueous secondary battery such as a lead storage battery or a lithium ion secondary battery is charged, the secondary battery is firstly charged with a constant current. And when the battery voltage of the secondary battery reaches the full charge voltage by the abovementioned constant current charging, the secondary battery is charged with a constant voltage which is set at the full charge voltage. Then, when the constant voltage charging time reaches a predetermined time, or when the charging current at the time of the abovementioned constant voltage charging becomes below a predetermined value, the charging of the secondary battery is ended.
The secondary battery is charged with a constant current and then with a constant voltage. However, according to such a charging method, several hours are required for charging a secondary battery to the full charge, and rapid charging cannot be achieved.
Therefore, a method for reducing charging time of a secondary battery is described in the Japanese Non-examined Patent Publication No. 2-119539 issued May 7, 1990. In this Japanese Non-examined Patent Publication, a method for charging a secondary battery is disclosed which comprises continuously charging the secondary battery with a constant current till the battery voltage of the secondary battery reaches a predetermined value above the fullcharge voltage, and then charging the secondary battery with a constant voltage set at the full charge voltage.
On the other hand, as described in the Japanese Non-examined Patent Publication No. 4-331425 issued Nov. 19, 1992, a lithium ion secondary battery is connected to a protecting circuit for protecting the secondary battery from being deteriorated due to overcharging. This protecting circuit detects the battery voltage of the secondary battery. And when the battery voltage is above a predetermined protecting voltage (which is set at a voltage, for example, somewhat higher than the full charge voltage of the secondary battery), the protecting circuit interrupts the charging of the secondary battery.
As abovementioned, according to a method for charging a secondary battery comprising firstly charging the secondary battery with a constant current till the battery voltage of the secondary battery reaches a predetermined voltage above the full charge voltage, and then charging the secondary battery with a constant voltage set at the full charge voltage, a secondary battery can be rapidly charged. However, according to a method for charging a secondary battery comprising continuously charging the secondary battery with a constant current till the battery voltage of the secondary battery reaches a predetermined voltage, the secondary battery is apt to be more or less deteriorated, and it is not desirable to continuously carry out such a charging method.
Further, according to the abovementioned method, when a secondary battery connected to a protecting circuit is charged, the protecting circuit operates to interrupt the charging before the battery voltage of the secondary battery reaches a predetermined voltage higher than the full charge voltage. As a result, rapid charging cannot be achieved.
Therefore, a first object of the present invention is to provide a method for rapidly charging a secondary battery connected to a protecting circuit without deteriorating the secondary battery and an apparatus for carrying out the method.
Further, a method for rapidly charging a secondary battery is described in the Japanese Non-examined Patent Publication No. 6-113474 issued Apr. 22, 1994. This method comprises charging a secondary battery with a constant current till the battery voltage reaches a second voltage (for example, 4.5 V) which is higher than a first voltage (for example, 4.1 V) set at the full charge voltage, and thereafter pulse charging the secondary battery with regulating to the second voltage, and then constant voltage charging the same with the first voltage so as to achieve rapid charging.
As abovementioned, a secondary battery such as a nonaqueous secondary battery is sometimes connected to a protecting circuit for protecting the secondary battery from being deteriorated due to overcharging. Such a case is described, for example, in the Japanese Non-examined Patent Publication No. 6-105457 issued Apr. 15, 1994. In this Japanese Non-examined Patent Publication No. 6-105457 issued Apr. 15, 1994, a protecting circuit is described, which stops charging of a secondary battery when the battery voltage becomes above a protecting voltage (for example, 4.2 V)which is higher than first voltage set at the full charge voltage of the secondary battery. As a result, even if a secondary battery is charged with a constant current till the battery voltage reaches a second voltage higher than a protecting voltage for the purpose of rapidly charging the secondary battery as abovementioned, the protecting circuit operates to stop the charging of the secondary battery before the battery voltage of the secondary battery reaches the second voltage. Consequently, rapid charging of the secondary battery cannot be achieved.
Therefore, a second object of the present invention is to provide a method for rapidly charging a secondary battery connected to a protecting circuit constructed to stop the charging of the secondary battery when the battery voltage of the secondary battery is higher than a first voltage which is the full charge voltage of the secondary battery for a predetermined time, with preventing the protecting circuit from stopping the charging of the secondary battery and an apparatus for carrying out the method.